The present invention relates to an improved method of determining, while drilling in the earth with a drill bit, the positions of geologic formations in the earth. More particularly, it relates to a method for improving the quality of a reference signal.
Conventional Reflection Seismology
Conventionally, reflection seismology utilized surface sources and receivers to detect reflections from subsurface impedance contrasts. The obtained image often suffered in spatial accuracy, resolution and coherence due to the long travel paths between source, reflector, and receiver. To overcome this difficulty, a technique commonly known as Vertical Seismic Profiling (VSP) was developed to image the subsurface in the vicinity of a borehole. With VSP, a source is suspended at a discrete borehole depth with a wireline and is activated to generate seismic signals. Field sensors are positioned at the surface to detect these seismic signals. Data is recorded and the process is repeated for several borehole depths. With the source positioned downhole, data can be acquired simultaneously at many surface locations with little more expense than for a single location. To reduce rig downtime, the drill bit was used as the source. However, because the drill bit signal is an uncontrolled pseudo-random process, a reference signal needs to be detected. Thus, in currently available methods of performing VSP, the signal generated by the drill bit travels up the drill string to the reference sensor and also propagates upward to the field sensors. By correlating the signal detected by the reference sensor with the signal detected by the field sensors, the travel time of the energy traveling from the drill bit to the field sensors may be determined. However, noise which is present in the reference signal will degrade the quality of the correlated signal.
Seismic While Drilling
One approach used in commercially available Seismic-While-Drilling (SWD) systems (e.g., TOMEX) to eliminate noise from the reference signal is through the use of an accelerometer 401 as the reference sensor. An example of a system utilizing an accelerometer is shown in FIG. 4. The signal generated by the drill bit 325 is used as a signal for seismic measurements and surface signals are recorded by an array of geophones 320 distributed around the drilling rig 301. The "reference signal", which is used for cross-correlation and following source signal detection, is recorded by an accelerometer 401 at the top of the drillstring 310. The accelerometer 401 has two sensors, one of which is sensitive to the noise but substantially insensitive to the acoustic signal transmitted up the drill string 310 from the source. The frequency band within which there is high coherence between the energy in the noise signal and the reference signal is determined. The noise signal is then amplified by a factor equal to the average ratio of the energy amplitude of the pilot signal to the noise signal within this frequency band, and this weighted noise signal is subtracted from the reference signal to reduce the noise in the reference signal. However, in many practical cases, the quality of the signal recorded at the top of the drillstring using an accelerometer is still very poor and still does not contain adequate information about the bit signature, especially for deviated extended reach wells. Often, even utilizing long term stacking (a method where, because the drill bit signal is not clear, the signal is averaged over a period of several minutes to extract a typical drill bit signature) does not improve the signal-to-noise ratio. Therefore, the results of SWD become unsatisfactory. Thus, there is a need to have a method for more accurately determining the reference signal.
Measurement-While-Drilling Telemetry
Measurement-While-Drilling (MWD) telemetry is a well known method for transmitting information between the bottom of the borehole and the surface. In MWD telemetry, information is transmitted by creating pressure pulses in the drilling fluid. This is performed by interrupting the flow of drilling fluid inside the pipe. At first glance, it might seem that the obvious solution to the problems of obtaining an improved reference signal lay in simply detecting the drill bit signal downhole near the drill bit and then transmitting this signal to the surface via MWD telemetry. However, MWD telemetry is limited to only a few bits per second. This is far below the rate of transmission needed to transmit to the surface the vast quantity of information needed in any representation of the waveform. Therefore, other approaches must be taken.
Improvement of Quality and Reliability of Seismic While Drilling (SWD) Measurements Due to Improvement of Quality of the Reference Signal
The present application discloses a method for making seismic while drilling (SWD) measurements by determining and analyzing the reference signal downhole near the drill bit and sending information about the signal to the surface using a limited number of transmission bits. In one embodiment, a library of anticipated drill bit wavelets is stored in memory downhole and in memory at the surface. This library of anticipated drill bit wavelets is based on long term experience (several years) in collecting drill bit signals downhole and, in fact, could also be considered a data base of these collected drill bit signals. The best matching wavelet is identified by the processor downhole and then a code identifying the wavelet and a scaling factor are sent to the surface. At the surface, the best matching wavelet is retrieved based on the code received and then a reconstructed signal is created using the retrieved wavelet and the scaling factor. In another embodiment, key characteristics of the signal such as central frequency, frequency band, etc., are calculated downhole and transmitted to the surface. These key characteristics are then used to reconstruct the reference signal which is then used for correlation of surface detected signals.
The disclosed innovations, in various embodiments, provide one or more of at least the following advantages:
allows reference signal information to be sent to the surface using a limited number of bits since measurement-while-drilling (MWD) telemetry from downhole to the surface is limited to a few bits per second, PA0 provides a better reference signal for cross-correlation purposes than is provided by present methods, and PA0 provides a real time determination of the reference signal much faster (in around 1 minute) than the current method of utilizing long-term stacking which may take as long as 20 minutes.